This invention relates to a pneumatic journal bearing suitable for application to a rotating shaft which acquires considerably high temperature during operation as in the case of a gas turbine shaft or an automotive turbocharger shaft, for example.
Pneumatic bearings, viz. gas-lubricated bearings, are used in various fields despite the need for the provision of a compressor for the feed of a pressurized gas, because pneumatic bearings have important advantages such as low friction torque, very smooth rotation and practicability at both high and low temperatures.
However, trouble arises in a pneumatic journal bearing if the shaft rotating in the bearing acquires high temperature and undergoes significant thermal expansion. For example, in an automotive turbocharger, the shaft connecting the turbine wheel and the compressor impeller is liable to undergo significant thermal expansion due to the conduction of considerable heat thereto from the turbine wheel which is driven by high temperature exhaust gases. Normally, the shaft as the journal in a pneumatic bearing is kept floating off the bearing surface by the action of compressed air introduced into the clearance space between the bearing surface and the shaft. However, the clearance space narrows as the shaft undergoes thermal expansion because the bearing part remains at a relatively low temperature without undergoing corresponding thermal expansion. It is not rarely that the clearance space becomes so narrow that the shaft makes sliding contact with the bearing surface by the influence of vibrations of the machine during its high speed operation. Then, wearing or seizing of the bearing and the shaft becomes a serious problem, and even breakage of the bearing or the shaft is probable in an extreme case.